Seismic Reflection acquisition and processing specifications

Data acquisition

The acquisition of seismic profiling data across the Cadell Fault was undertaken during two separate surveys (both are included in GA seismic survey number L169). The initial survey was designed to acquire sufficiently high-resolution data to be able to decide on which of two separate locations to site a trench across the fault. The second survey was designed to acquire very high-resolution data to refine the locations of possible trench sites identified by the first survey. A total of six traverses were recorded.
 
The source used for the first survey was the Australian National Seismic Imaging Resource (ANSIR) IVI T1500 Mini Vibrator operating in P-wave mode (ANSIR, 2004). A filtered ground force pilot signal was used to control the source. Each source vibration comprised four 10-second sweeps with a frequency of 10-240 Hz and 0.2 second taper which were correlated and then stacked. The shot points were located every 10 metres, coincident with every receiver location, and were offset orthogonal to the line by approximately 2 m. Additional shots were located at offsets of up to 1640 metres to record reversed refraction data. The data were recorded on a 24-channel StrataVisor seismic acquisition system in SEG-Y format. The listening time was 2 seconds, and the recording sample interval was 1 millisecond. The gain level was 24 dB, with no filter applied. The receivers comprised groups of four 10 Hz geophones deployed at 10 metre intervals. The spread configuration employed was a combination of split-spread and off-end. The 24 channel spread cable was advanced half a cable length after each 24 shots and the last twelve shots of the line were repeated, thus providing 24-fold coverage. 
A hammer source was used for the second survey. The shot points were located every 2 or 1 metres, coincident with every receiver location. Additional shots were located at offsets of up to 190 metres to record reversed refraction data. The data were recorded on a Seistronix RAS-24 seismic acquisition system in SEG-2 format. The record length was 1 second, and the recording sample interval was 0.5 millisecond. The receivers were single 8 Hz geophones deployed at 2 or 1 metre intervals. The spread configuration employed was a combination of split-spread and off-end and provided a maximum 48-fold coverage. 
The acquisition parameters are tabulated in Supplementary Table 1.

Data processing

The seismic data were processed using Disco-Focus processing software. The key processing steps were:

Editing – to remove off-end shots (> 240 m offset), the trace for the receiver next to the Minivibe, noisy traces, and to correct polarity. 	 

Refraction and Elevation Statics – to remove effect of near surface low velocity layer and elevation differences. Static corrections were made for elevation differences above 0 m (base of scarp) and variable delay time in near surface layer (V = ~650 m/s). First break picks were made on the first peak on the correlated Minivibe records. A single refractor model solution used the ~1800 m/s branch to determine delay times to the base of the ~600 m/s layer, and hence calculate refractor depth and resultant statics (with a replacement velocity of 1750 m/s between base of low velocity layer and datum). A later branch (V > 3000m/s) was not included in the solution. Much better sections were obtained with refraction (and elevation) statics than with elevation statics alone. A marked improvement of the hammer seismic sections was obtained by using refraction statics (and stacking velocities) from the coincident Minivibe line. This improved continuity on the sub-horizontal reflector beneath the scarp. Automatic residual statics were calculated after NMO correction, but were generally less than 1 or 2 ms and did not bring about significant improvement.

Spectral Equalisation – to suppress low frequency noise and enhance higher frequency reflections. Spectral equalisation minimises contamination by low-frequency source-generated noise, such as ground roll and direct waves, plus random noise such as traffic noise. This step is particularly critical at shallow depths where the fold is lower and where source generated noise interferes with reflections. Spectral equalisation performs spectrum balancing or whitening across the range of signal bandwidth. The spectrum balancing was carried out using 10 overlapping gates of 12 Hz width with the first being 36-40-44-48 Hz. An AGC within the module was chosen to have a gate width of 250 milliseconds, which gave better results for reflector amplitudes than wider gates.

Creation of CDP “super gathers” – to progressively combine adjacent CDP gathers. After sorting the traces into CDP gathers, gathers were combined into “supergathers” equivalent to a running mix, i.e. traces from 2001 and 2002 formed new gather 2001, traces from 2002 and 2003 formed new gather 2002, and so on. This doubled the fold in each gather to facilitate stacking velocity analysis and to improve the final stack.

Stacking velocity analysis  - CDP supergathers for velocity analysis (after spectral equalisation and application of refraction statics) were chosen on the basis of reflector quality. In the case of line 04ga- cf2 in particular, this meant choosing gathers incorporating longer offsets where reflections were more obvious. (Because conventional roll-along acquisition was not possible, the offset distribution in the CDP gathers changed systematically along the line).Velocity analysis was carried out approximately every 25 CDPs (Note that a step of +50 ms was applied prior to statics, to approximate surface referenced velocity analysis – this was removed in the very last stages of processing prior to final display). Due to the relatively short offsets, stack quality was insensitive to velocity below 500 ms. In the vicinity of 200 ms, the stacking velocity was remarkably consistent at ~1500 m/s and variations of +/- 100 m/s degraded stack quality. In the very shallow section, velocities were difficult to determine due to low fold and near-source noise.

NMO Correction, Alpha-trimmed Mean Stack, Stretch Mute – to remove moveout, high and low amplitudes, NMO distortion and  first breaks prior to stacking. Normal moveout (NMO) correction was applied using the velocity functions from velocity analysis. Instead of a linear stack of traces across the CDP supergathers, an alpha trim mean was used whereby 50% of the data was discarded at each sample point, that is, 25% of samples at the high amplitude end and 25% at the low end. This stacking technique is superior in areas contaminated with non-Gaussian noise, such as near source noise. In order to remove data distorted by NMO correction and also to mute first arrivals, a stretch mute of 20% was employed.

Post stack time migration – to migrate reflectors to correct position in section and collapse diffractions (finite difference in f-x domain). Post stack time migration was performed, with the aim of collapsing any diffractions and moving dipping reflections into their correct positions in order to sharpen the image of the fault. The MIGFX operator is based on a finite-difference approximation to the monochromatic wave equation. It can handle velocity variations in both the lateral and vertical directions. The stacking velocity functions were used for the migration velocities.

Coherency enhancement – to enhance coherent reflections. A signal enhancement algorithm (DIGISTK) was applied for final display only to enhance events that are coherent across several traces, thus making reflections stand out better against background noise. This process determines the coherency of linear events for all dips within a specified range and outputs a second set of seismic traces that is essentially a coherency section. A weighted sum is then made of the coherency section and the actual seismic section, such that a weight of zero reproduces the input seismic data and a weight of one gives the coherency section. For final display of the Minivibe sections, weights of 0.5 were used (0.75 for the hammer line). 

Supplementary Table 1: Seismic Acquisition Parameters
			
Survey Number	L169	L169	
Line Number	04ga-cf2	05ga-cf1	
Area	Echuca - Deniliquin	Echuca - Deniliquin	
Location  - Start of line	35.97782S, 144.88237E	35.597417S, 144.905274E	
Location  - End of line	35.98062S, 144.90025E	35.600034S, 144.904908E	
Elev. (relative) at start (m)	0	-6.3	
Elev. (relative) at end (m)	-14.13	-1.232	
Source	MiniVibe, IVI T1500	10 lb hammer	
Source parameters	Filtered Ground Force pilot signal; Correlate before stack; 24 dB gain	Steel ground plate (first day); no plate (second day)	
Acquisition System	StrataVisor 	Seistronix RAS-24	
Channels	24	96	
Geophones (Hz)	10	8	
Geophones per station	4	1	
Source interval (m)	10	2	
Receiver interval (m)	10	2	
Spread configuration	Split spread & off-end	Split spread & off-end	
Maximim offset (m)	1190	190	
Fold	24	48	
Record length (s)	2	1	
Sample interval (ms)	1	0.5	
Line length (m)	1190	286	
Type (P/S)	P	P	
Sweep (Hz)	10-240	N/A	
Sweep length (s)	10	N/A	
Number of sweeps/shots	4	1	
Filter (Hz)	Nil	Nil	
Recording Format	SEG-Y	SEG-2	
Velocities (m/s)	1000 - 4000 ?	 -	
Target depth	10 - 250 m	 -